Wide-band magnetic yoke deflection system

ABSTRACT

A system for providing current to an inductive load such as a magnetic yoke deflection system is improved by the addition of a low power, high frequency amplifier and a few passive components. Deflection current components at frequencies higher than the frequency spectrum of the unimproved magnetic yoke deflection system are provided by the high frequency amplifier, thereby enhancing the deflection system bandwidth far beyond limitations encountered using the teachings of the prior art.

United States Patent 1 1 1111 3,725,727 Waehner [4 1 Apr. 3, 1973 54] WIDE-BAND MAGNETIC YOKE 3,408,586 10/1968 Ordower ..330/l26 DEFLECTION SYSTEM Glenn C. Waehner, Scarsdale, NY.

United Aircraft Corporation, East Hartford, Conn.

Feb. 8, 1971 Inventor:

Assignee:

Filed:

Appl. No.:

US. Cl .Q. ..315/27 TD, 330/l26 Int. Cl ..H01j 29/70 Field of Search ..3l5/27 R, 27 TD; 330/126 References Cited UNITED STATES PATENTS 3,529,206 9/1970 Rodal 1 5/27 TD Primary ExaminerBenjamin R. Padgett Assistant Examiner-J. M. Potenza Attorney-Melvin Pearson Williams [57] ABSTRACT A system for providing current to an inductive load such as a magnetic yoke deflection system is improved by the addition of a low power, high frequency amplifier and a few passive components. Deflection current components at frequencies higher than the frequency spectrum of the unimproved magnetic yoke deflection system are provided by the high frequency amplifier, thereby enhancing the deflection system bandwidth far beyond limitations encountered using the teachings of the prior art.

1 Claim, 2 Drawing Figures WIDE-BAND MAGNETIC YOKE DEFLECTION SYSTEM BACKGROUND OF THE INVENTION 1. Field of Invention This invention relates to cathode ray tube deflection circuitry, and more particularly to a wide-band magnetic yoke deflection system.

2. Description of the Prior Art CRT displays employing magnetic yoke deflection commonly use one of two techniques to drive the yoke. A non-dissipative and inexpensive system currently in use is the resonant energy recovery circuit. This circuit has wide application where a raster is being generated, such as in commercial television, but no application to display systems where deflection is derived from a stroke generator. A high degree of display linearity is not possible with a resonant energy recovery circuit, primarily because it is an open loop type of circuit.

A more flexible system employs a feedback amplifier I where current in the yoke is sampled by feeding it through a small sampling resistor and feeding back the resulting voltage for comparison with the voltage input. Within the limitations imposed by circuit bandwidth, the current waveform in the yoke will be a faithful reproduction of the voltage waveform at the circuit input. For a stroke written display or a display where a very precise raster must be generated, this property is essential. Since deflection yokes commonly require peak currents of several amperes, a decisive factor in display bandwidth is the frequency response of the output stage required to drive the yoke. In the present state of the art, it is a common experience to find that transistors having high current and high voltage capability have a low bandwidth. The usual consequence is that for high quality displays, the output stage is very expensive and there is a tendency toward circuit instability due to the low frequencies at which frequency dependent phase shift and attenuation are introduced by the output stage.

f yoke deflection circuits thus far devised, either their usage is limited to specialized application, they are of restricted bandwidth, or they employ components which are extremely expensive.

SUMMARY OF INVENTION The primary object of this invention is to provide an improved system for driving an inductive load.

Another object of the present invention is to provide a simple and improved yoke deflection system which has wide bandwidth.

According to the present invention, the addition of an inductor in series with an inductive load, such as a deflection yoke, and a low power, high frequency amplifier stage, increases the bandwidth of a conventional system for driving an inductive load.

The high frequency amplifier stage augments the power amplifier of a yoke deflection system known to the prior art, so that the bandwidth of the improved yoke deflection system is limited only by the upper pass frequency of the high frequency amplifier stage. The present invention widens the bandwidth of inductive load amplification systems significantly more than heretofore possible or economically feasible.

Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of a preferred embodiment thereof as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic block diagram of a preferred embodiment of the present invention; and

FIG. 2 isan illustration of the frequency response of elements in the embodiment of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, a prior art magnetic yoke driving circuit is properly illustrated by connecting the output of a power amplifier 10 to an inductive load (in this embodiment, a magnetic deflection yoke 12) and excising an improvement 14 of the present invention. The current through the magnetic deflection yoke 12 is returned to ground through a very small sampling resistor 16 (typically 1 ohm or less) thereby causing a voltage waveform, at a voltage sampling point 17, proportional to the current waveform through the magnetic deflection yoke 12. This voltage is applied to a summing resistor 18 which is connected to a summing point 20. An input voltage to the yoke deflection system, proportional to the .desired deflection, is applied at terminals 22 to an input summing resistor 24 connected to the summing point 20, which is the input to a high gain voltage amplifier 26, whose output drives the power amplifier 10.

The cascade combination of the voltage amplifier 26 and the power amplifier 10 must supply a phase reversal to cause negative polarity of feedback from the voltage sampling point 17 to the summing point 20; in this embodiment, this is provided by the voltage amplifier 26. Any tendency for the potential at the voltage sampling point 17 to vary (variation in yoke current) away from the value required for system equilibrium (imposed by the input signal voltage at terminals 22), will be reduced by the negative feedback. The transconductance of the cascade combination of the voltage amplifier 26 and the power amplifier 10 should be sufficiently high so that the voltage at the summing point 20 will be substantially zero, and variations from system equilibrium at the voltage sampling point 17 which would otherwise cause deviation of the summing point 20 from zero, will immediately be reduced to a negligible value. This virtual ground at the summing point 20 is a well-known phenomena where amplifier feedback circuitry is used, and causes the current through the input summing resistor 24 to be the input signal voltage (at terminals 22) divided by the value of the input summing resistor 24. Since the voltage amplifier 26 must draw no current into its input, the current through the input summing resistor 24 must be equal to the current through the summing resistor 18, thereby determining the potential at the voltage sampling point 17, and the yoke current (which is the voltage at the voltage sampling point 17 divided by the value of the sampling resistor 16). The amplifier characteristics hereinbefore described cause the voltage across the sampling resistor 16 and therefore the current through the sampling resistor 16 (current through the magnetic deflection yoke 12) to be proportional to the input signal voltage at terminals 22, with the current amplitude scaled by the ratio of the summing resistor 18 to the input summing resistor 24, at frequencies in the pass band of the magnetic yoke deflection system. Because of the feedback and the virtual ground described hereinbefore, the output of the power amplifier 10 is independent of the gain of amplifiers 10, 26.

The closed loop bandwidth of the prior art magnetic yoke deflection system is dependent upon the cascade frequency response of the voltage amplifier 26 and the power amplifier 10 (illustration a, FIG. 2), where the frequency response is the ratio of voltage at the sampling point 17 to the voltage at the summing point 20, as a function of frequency. Because of negative feedback, the magnetic yoke deflection system bandwidth extends to approximately the zero db crossover frequency of the cascade frequency response where, because of low gain, the voltage of the summing point 20 departs significantly from zero and the closed loop magnetic deflection system departs significantly from its midband value. The improvement 14 of the present invention, which can be suitably adapted to the prior art, extends the closed loop bandwidth of the magnetic yoke deflection system to a much higher frequency.

In accordance with the present invention, the improvement 14 comprises a high frequency amplifier 32 driven by the voltage amplifier 26, with its output connected to a junction 36. The junction 36 is connected to the magnetic deflection yoke 12, and to the power amplifier 10 through an inductor 34. Because the invention is in the configuration of a high gain feedback amplifier, midband yoke current, in response to the input signal at terminals 22, is determined by the value of the sampling resistor 16, scaled by the summing resistors 18, 24, and is independent of the gain of the high frequency amplifier 32 (as well as the amplifiers 10, 26, as described hereinbefore).

The high frequency amplifier 32 is composed of devices with a low output current capacity, incapable of supplying the high yoke current demands common at the lower frequencies. Therefore, it is necessary to select the gain for the high frequency amplifier 32, the gain for the power amplifier 10, and the inductance of the inductor 34 so that at low frequencies, in the pass band of the power amplifier 10, the high frequency amplifier 32 does not supply any current.

In the absence of the high frequency amplifier 32, the yoke 12 and the inductor 34 form a voltage divider; all of the current through the magnetic deflection yoke 12 passes through the inductor 34. The gain of the high frequency amplifier 32 is adjusted so that, at frequencies in the pass band of the power amplifier 10, an input (common to the power amplifier 10 and the high frequency amplifier 32) will cause a voltage at its output equal to the voltage at the junction 36, formed by the voltage divider; because of this, connecting the output of the high frequency amplifier 32 to the junction 36 will not cause a change in the voltage output of the power amplifier 10 or the voltage at the junction 36, and therefore, in the pass band of the power amplifier 10, all of the yoke current will pass through the inductor 34 from the power amplifier l and none of it will come from the high frequency amplifier 32.

Accordingly, the output voltage, V of the power amplifier is given as:

where K low frequency voltage gain of the power amplifier l0; and V input voltage into the power amplifier 10.

The voltage, V at the junction 36 is given as:

where L inductance of the magnetic deflection yoke 12, and L inductance of the inductor 34. Therefore, the midband gain, G, of the high frequency amplifier 32, in the pass band of the power amplifier 10, must be:

At frequencies above where the response of the power amplifier It) begins to deteriorate, (illustration a, FIG. 2), the current not supplied by the power amplifier 10 is supplied by the high frequency amplifier 32 (illustration b, FIG. 2). Outside of the pass band of the power amplifier 10 (within the bandwidth of the high frequency amplifier 32), the feedback characteristics of the magnetic yoke deflection system are determined by the cascade frequency response of the voltage amplifier 26 and the high frequency amplifier 32.

According to the invention, all of the feedback amplifier characteristics hereinbefore described for the voltage amplifier 26 in cascade with the power amplifier 10 within the pass band of the power amplifier 10, apply to the voltage amplifier 26 in cascade with the high frequency amplifier 32 in the range of frequencies above the pass band of the power amplifier 10, but within the bandwidth of the high frequency amplifier 32. This is obviously true because, even in the complete absence of the power amplifier 10, the remaining elements are in the configuration of the prior art, thereby supplying yoke current at frequencies above the pass band of the power amplifier 10. The inductor 34 degenerates into a minor load on the output of the high frequency amplifier 32, but draws little current because this condition occurs at a relatively high frequency, and is of no importance since none of the current drawn effects the drop across the sampling resistor 16 or current through the yoke.

An essential feature of the invention is that the inductive load driving system upper pass frequency is increased from the upper frequency determined by the pass band of the power amplifier 10 to the upper frequency determined by the pass band of the high frequency amplifier 32.

Another feature of the invention is that, in all known CRT display applications, yoke deflection current requirements are relatively low in a spectrum beyond the bandwidth of the inexpensive power amplifier 10, and the high frequency amplifier 32 may be implemented with inexpensive low power components since its current requirements are similarly low.

Although the invention has been shown and described with respect to an exemplary embodiment thereof, relating to magnetic deflection yokes, it should be understood by those skilled in the art that the invention is applicable to any other similar inductive load driving application, and that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Having thus described a typical embodiment of my invention, that which I claim as new and desire to secure by Letters Patent of the United States is:

1. In a system for driving a magnetic deflection yoke connected to the output of amplification means having a voltage amplifier driving a power amplifier, said amplification means responsive to the difference between an input voltage representative of desired yoke current and a feedback voltage representative of actual yoke current, the improvement comprising:

an inductive reactance, said power amplifier connected to said yoke through said inductive reactance; and

a high frequency amplifier driven by the voltage amplifier, having its output connected to the junction of said yoke and said inductive reactance and, at frequencies within the pass band of said power amplifier, having a gain related to the gain of the power amplifier by the ratio of the yoke inductance to the total inductance of said yoke and said inductive reactance. 

1. In a system for driving a magnetic deflection yoke connected to the output of amplification means having a voltage amplifier driving a power amplifier, said amplification means responsive to the difference between an input voltage representative of desired yoke current and a feedback voltage representative of actual yoke current, the improvement comprising: an inductive reactance, said power amplifier connected to said yoke through said inductive reactance; and a high frequency amplifier driven by the voltage amplifier, having its output connected to the junction of said yoke and said inductive reactance and, at frequencies within the pass band of said power amplifier, having a gain related to the gain of the power amplifier by the ratio of the yoke inductance to the total inductance of said yoke and said inductive reactance. 